The present invention relates to a method of processing a light sensitive printing plate precursor, and particular to a method of processing a light sensitive printing plate precursor, which can reduce sludge produced during processing and provide a printing plate preventing staining occurring at non-image portions.
A light sensitive planographic printing plate precursor (hereinafter referred to also as a printing plate precursor) is known which comprises a support subjected to hydrophilization treatment and provided thereon, a photopolymerizable layer and a protective layer. Recently, in order to obtain a printing plate with high resolution image in a short time or to carry out a direct plate making system, a method has been applied in which a planographic printing plate precursor is digitally exposed based on image information employing laser rays, and developed with a developer to prepare a printing plate.
As one embodiment, there is a system preparing a planographic printing plate in which a light sensitive planographic printing plate precursor is scanning-exposed employing a light source modulated with an image signal transmitted by communication lines or output signals from an electronic plate making system or an image processing system.
Generally, a photopolymerizable layer contains an acryl monomer, an alkali soluble resin, a photo-initiator, and optionally a sensitizing dye to absorb a range of wavelengths of emitted light (particularly laser light).
As a light source for exposing a photopolymerization type light sensitive planographic printing plate precursor is used a visible light source having a longer wavelength such as an Ar laser (488 nm) or an FD-YAG laser (532 nm). In recent years, semiconductor lasers employing, for example, InGaN type material, which can continuously emit light with a wavelength of from 380 to 430 nm, are about to be put into practical use. A scanning exposure system employing light with such a short wavelength as a light source has advantages in providing sufficient output power and an economical system, since the semiconductor lasers can be structurally manufactured at low cost. Further, a planographic printing plate precursor to be applied to the above light source has a spectral sensitivity shorter than that of a planographic printing plate precursor to be applied to a system employing a conventional FD-YAG laser or Ar laser, and therefore, operation under safe light is easy.
A photopolymerization type light sensitive planographic printing plate precursor is imagewise exposed, optionally heated, washed with water (in order to remove a protective layer), developed with a developer to remove unexposed portions, washed with water, and post-processed with a finisher or a gumming solution to make non-image portions hydrophilic. Thus, a printing plate is obtained.
As a support of the light sensitive planographic printing plate precursor, one having high hydrophilicity, high water retention property, and excellent adhesion to the light sensitive layer is desired, and an aluminum plate whose surface is subjected to graining for roughness is generally used.
A light sensitive planographic printing plate precursor with high sensitivity reduces the time required for exposure, and can shorten the time necessary to prepare a printing plate. When printing is carried out employing a printing plate obtained from such a planographic printing plate precursor, printing is often suspended on account of registering or a break. When printing, after suspended, is resumed, minute stain spots (hereinafter referred to as peppering) may occur at the non-image portions of the printing plate. In order to prevent occurrence of the peppering, a method has been desired which maintains hydrophilicity of the non-image portions of the printing plate.
As a developer for a light sensitive planographic printing plate precursor, an aqueous solution of an alkali metal silicate is generally used, and the alkali metal silicate reacts with the aluminum plate support to increase hydrophilicity of the aluminum plate support surface. However, this method cannot sufficiently maintain hydrophilicity of the plate surface, and cannot completely prevent occurrence of the peppering.
A photopolymerization type light sensitive planographic printing plate precursor has high sensitivity, and can provide a printing plate in a short processing time. However, it has been found that when the photopolymerization type light sensitive planographic printing plate precursor is repeatedly developed with a developer replenished with a developer replenisher, employing an automatic developing machine, components in the photopolymerizable layer of the printing plate precursor are eluted into the developer, and then precipitate. The photopolymerization type light sensitive planographic printing plate precursor, which forms a latent image due to free radical reaction, has high sensitivity, but has drawback in that the free radical reaction is inhibited by oxygen. As a countermeasure, it is well known that it is effective to provide an oxygen-shielding layer on the photopolymerizable light sensitive layer, and to incorporate polyvinyl alcohol as a main component in the oxygen-shielding layer. However, when a light sensitive planographic printing plate precursor comprising polyvinyl alcohol as a main component is repeatedly developed with a developer replenished with a developer replenisher, employing an automatic developing machine, it has been found that an agar-like gel is produced in the developer of the developer tank of the automatic developing machine, resulting in development faults. Therefore, a developer, which reduces occurrence of sludge occurrence and lightens cleaning load of the developer tank, has been sought.
Pre-washing before development can prevent incorporation of the oxygen-shielding layer components into the developer. However, when washing water is used in the pre-washing step and discarded without being reused, a large amount of washing water is required, and when washing water is used while circulated as a water saving measure, washing water in the pre-washing step is gradually polluted, resulting in reduction of the detergent function.
The present invention has been made in view of the above. An object of the invention is to provide a light sensitive planographic printing plate precursor processing method of providing a planographic printing plate which prevents peppering from occurring at non-image portions of the printing plate, when printing is started employing the printing plate, suspended on account of registering or a break, and then resumed. Another object of the invention is to provide a light sensitive planographic printing plate precursor processing method of reducing sludge produced in a developing tank during processing and lightening cleaning load of the developer tank.